1. Field of the Invention
The present invention relates to a data transmission apparatus and a method for transmitting moving picture data, and more particularly, to a moving picture data transmission apparatus and a method for transmitting so as to enable playback of coded moving picture data immediately at a plurality of receiving terminals.
2. Description of the Related Art
Recently, with the spread of ADSL (Asymmetric Digital Subscriber Line), FTTH (Fiber To The Home) and the like, the broadbanding of computer networks has proceeded rapidly. Further, the processing capability of various computing devices, beginning with the personal computer (PC), continues to improve.
Against this backdrop of improvement in the performance of the communication environment as well as the data processing environment, streaming technology that distributes multimedia data, such as moving picture data and audio data, in real time over networks has been commercialized. With this streaming technology, the user can enjoy broadcasted live media or pre-recorded media, enjoy pre-recorded media on demand, and so forth.
Generally, communication methods called multicast and unicast are used in the distribution of this type of video content. Multicasting is a technology for transmitting one packet to a specified number of receiving terminals, and a multicast packet is copied by the appropriate router along the transmission path and transmitted to the receiving terminals. By contrast, unicasting is one-on-one communication, and when transmitting to multiple terminals it is necessary to copy the packet on the sending terminal end and specify each and every receiving terminal, although it is possible to transmit data to a specified number of terminals.
In addition, as a data transmission method for distributing moving picture and audio multimedia data over a network, RTP (Real-time Transfer Protocol) is known. RTP is defined as RFC1889 and RFC1890 by the IETF (Internet Engineering Task Force), and is a protocol for data transmission of audio and video in real time.
Multimedia data, in particular moving picture data, can be too large in size to be transmitted as is, and therefore it is usually compression-encoded before distribution. A variety of efforts is being undertaken in order to achieve high data compression rates while minimizing deterioration in picture quality, of which the MPEG-2 format and the MPEG-4 format standardized by the ISO (International Organization for Standardization) as international standards are well-known representative coding technologies.
In these MPEG compression coding formats, prediction coding is applied to the frames that make up the moving picture. Depending on the type of prediction coding applied, the frames are divided into one of three types: I frame, P frame or B frame. An I frame is an intraframe picture encoded within the frame, a P frame is an interframe forward prediction coded picture that uses the past picture frame prior to the frame that is to be encoded, and a B frame is an interframe bidirectional prediction coded picture that uses both the previous and the next picture frames. Of these three types of coded frames, only the I frame can be decoded without using information from another picture frame.
When distributing moving pictures encoded using these types of prediction coding over a network, unless data from the I frame is received the moving pictures cannot be properly decoded and played back at the receiving end. As a result, it is necessary to start transmission to the receiving terminal from an I frame. However, when streaming live video, the moving picture frame that can be transmitted when a request for distribution is received from the receiving terminal is not always an I frame. If no request for distribution comes when it comes time to transmit an I frame, it is not possible to start transmission until the next I frame is coded. Otherwise, the picture cannot be decoded on the receiving end even if transmitted. The end result is that a delay occurs at the receiving terminal until playback starts.
Temporarily switching the coding format to intraframe coding when there is a new request for distribution in an apparatus that transmits stream data by unicast is disclosed in JP-A-2002-305733.
In addition, in JP-A-H05-252511, in a case in which the transmission apparatus is routinely carrying out both intraframe coding and interframe coding and there is a new request for distribution during transmission, carrying out transmission from the I frame only for the receiving terminal that originates that request is proposed.
With the method that is proposed in JP-A-2002-305733, when a new receiving terminal is added during distribution that is already in progress (hereinafter such a receiving terminal is referred to as a latecomer terminal), the coding format switches to intraframe coding. By doing so, however, although it makes possible playback without delay at the receiving terminal, an I frame is also transmitted to the existing terminals as well. Generally, the compression efficiency for intraframe coding is lower than that for interframe coding and the data volume is greater, and consequently, the addition of a latecomer terminal results in additional communication band consumption.
Moreover, the system proposed by JP-A-H05-252511 involves a technique of routinely conducting both intraframe coding and interframe coding and carrying out transmission of an I frame only for the latecomer terminal. As a result, either a special coding circuit that routinely carries out both types of coding or two coding circuits—a coding circuit for carrying out intraframe coding and a coding circuit for carrying out interframe coding—are required, which becomes very expensive.